The invention relates to an apparatus for refractive surgery, to a control program for such an apparatus, and to a method for generating such a control program.
“Refractive surgery” is understood by specialist circles as the alteration of the imaging properties of the optical system “eye” through laser radiation. The laser radiation thus alters the refractive properties of one or more components of the eye. Since it is mainly the cornea that determines the imaging property of the eye, refractive surgery is used, in particular, to perform shaping of the cornea.
A prominent example of such reshaping of the cornea for the purpose of altering its refractive properties is that of LASIK. The present invention relates, in particular, to the LASIK technique. Moreover, the invention is also generally applicable in PRK and EPI-LASIK. The invention can also be applied in the case of use of femtosecond lasers.
In the case of LASIK, according to the prior art, a so-termed ablation profile is determined, i.e. it is calculated, on the basis of measurements on the eye and, if appropriate, other influencing variables, how much tissue (stroma) is to be removed at which location of the cornea, in order that, following the removal, the cornea has an optimum shape for the eye to be treated, i.e. the previously existing optical imaging defects of the eye are, as far as possible, corrected. Various methods are known in the prior art for the said calculation of the ablation profile.
Once the ablation profile has been determined for the eye that is to be treated, it is then calculated how this profile can best be removed (ablated) from the cornea by means of laser radiation. For this purpose, there is calculated a sequence of individual laser pulses, in space and time, which, in interaction with the stroma, effects the required reshaping of the cornea.
The ablation profile is a three-dimensional shape, and corresponds to a volume of the cornea that is to be removed.
The means of so controlling laser radiation by means of a computer that a predefined ablation profile is removed are well known as such in the prior art.
In the case of execution of refractive surgery, the computer controls the laser radiation, for example individual laser pulses (“spots”), over the eye in a space and time sequence in accordance with a control program.
In the case of this control of the laser radiation in respect of the eye, a quite determinant reference quantity is the so-termed ablation centre. The ablation centre is the spatial reference point to which the said spatial sequence of laser pulses relates. In the prior art, the mid-point (the centre) of the pupil is usually used as the ablation centre. The pupil, i.e. the opening left open by the iris, as a diaphragm, for the passage of radiation into the eye, has a relatively sharp contour, and therefore it is suitable for recording by means of a camera and analysed by means of image processing programs. Such recording devices and processing programs are well known as such in the prior art, and the present invention can have recourse thereto to some extent.
During refractive surgery, the eye to be treated is not of a constant size, but rather properties and also the orientation of the eye can change during the procedure. Changes in the orientation of the eye are tracked, according to the prior art, by means of a so-termed eye tracker. An eye tracker traces movements of the eye, usually through the aforementioned recording of the pupil by means of a camera and with subsequent image processing. Movements of the eye are also executed concomitantly by the pupil, and therefore the movements can be determined in this manner, and the control of the laser beam can be matched to such eye movements, i.e. the previously calculated ablation profile is removed with precision, despite eye movements during the operation, which, as a rule, cannot be precluded in a reliable manner.